1. Field of the Invention
This invention is generally related to analysis of subterranean formations, and more particularly to measurement of the speed of sound in a downhole fluid in order to facilitate acoustic logging operations for formation evaluation and reservoir characterization.
2. Background of the Invention
Wireline and logging-while-drilling (LWD) tools are used to measure physical, chemical, and structural characteristics of formations surrounding a borehole. For example, data gathered by logging tools can be used to interpret formation stratigraphy, lithology, mineralogy, and pore fluid content. Logging tools typically emit one or more of an acoustic, electromagnetic and optical signal, and measure the response to that signal. In the case of acoustic logging tools, changes in amplitude, phase and speed can be utilized to characterize the formation. Some acoustic logging tools utilize modal propagations, such as Stoneley, dipole and quadrupole modes, to measure formation compressional and shear speeds. Examples include measurement of formation shear speed from borehole flexural (a.k.a. dipole) modes in wireline logging, extraction of formation shear speed from borehole quadrupole mode in logging while drilling, and to a lesser extent extraction of formation compressional speed from “leaky” fluid modes. These acoustic logging tools operate based on the dependence of the speed of the borehole modes on formation acoustic properties. However, the speed of the modes is also dependent on the acoustic speed of downhole fluids such as borehole mud. It is therefore desirable to measure independently the sound speed of downhole fluids in order to more accurately characterize the formation. Characterization of downhole formation fluid is of great interest, since it is often the motivation of the entire drilling activity.
A wide variety of equipment is available to measure sound speed of fluids outside the borehole environment. However, measuring the sound speed of downhole fluids outside the borehole can be problematic because the sound speed of a fluid is a function of its constituents, temperature and pressure. Since temperature and pressure tend to change when a borehole fluid is transported to the surface, and in addition, the fluid constituents, temperature and pressure at a given location in the borehole may also change over the time of drilling and production of the well, it would therefore be desirable to have a better technique to measure the sound speed of downhole fluids.